The present invention relates generally to cutting tools, and more particularly to a composite drill bit which employs an adhesive to facilitate the rigid attachment of a drill blank to a shank.
There is currently known in the prior art drill bits for use in differing applications which comprise a shank having a cutting tool rigidly secured thereto. More particularly, the shank of the prior art drill bits includes a hole extending therein which accommodates a portion of the cutting tool. In the prior art drill bits, two different processes or techniques are typically employed to facilitate the rigid attachment of the cutting tool to the shank. The first prior art process is a brazed slip fit process wherein the cutting tool, subsequent to being inserted into the shank hole, is rigidly attached to the shank itself via a low temperature braze alloy which is used as a bonding filler material in a clearance provided between the cutting tool and the shank. The second process is a heat shrinking or shrink fit process. The heat shrinking process involves fitting a cutting tool of slightly larger outside diameter into the smaller shank hole of the shank. In this respect, heat is applied to the shank at a temperature sufficient to thermally increase the diameter of the shank hole thereof to one which is greater than the diameter of the cutting tool. Subsequent to the thermal expansion of the shank hole, the cutting tool is advanced thereinto. The shank is then allowed to cool, thus resulting in the thermal contraction thereof about the cutting tool, and hence the rigid attachment of the cutting tool to the shank through an interference fit.
Though the above-described processes are generally suitable for facilitating the rigid attachment of the cutting tool to the shank, they possess certain deficiencies which detract from their overall utility. More particularly, in the prior art heat shrinking process, the strength of the shrink fit depends on the values of the coefficients of friction for the two materials and the pressure between the shank and the cutting tool. Shrink or interference fits typically have only a twenty percent (20%) to forty percent (40%) effective contact area due to surface irregularities of the parts in contact. The pressure between the parts is a function of actual interference therebetween, and will vary due to part tolerances and the reduced contact area. Additionally, shrink or interference fits are based on residual stresses in the components which can lead to premature fatigue failure of the assembly. Because the effective contact area between the parts is relatively low when the heat shrinking process is employed, the assembly is highly susceptible to failure during a cutting operation which results in the pull-out of the cutting tool from within the shank.
The prior art brazed slip fit process is itself undesirable due to such process sometimes giving rise to occurrences of radial distortion of the cutting tool relative to the shank. This process also results in the annealing of the shank which softens the same and thus increases its susceptibility to failure. Moreover, the prior art brazed slip fit process is both time consuming and labor intensive, and thus costly. The present invention overcomes the deficiencies of these prior art processes by providing a method of assembling a composite drill bit wherein an insert, such as a drill blank, is rigidly attached to a shank member through the use of an ambient cure high strength, high temperature adhesive.